1. Field of the Invention
The present invention relates to a friction material composition suitable for the production of friction materials such as disc brake pads or brake linings to be used to brake cars, and a friction material using the same.
2. Description of the Related Art
Friction materials such as disc brake pads and brake linings are used to braking cars. Friction materials mainly used at present are Non-Asbestos Organic (hereinafter abbreviated to as “a NAO material”) series disc brake pads, which, as disclosed in Japanese Unexamined Patent Publications No. Hei. 2-132175A and No. Hei. 6-184525A, contain a friction regulator including fibrous materials such as Aramid fiber, inorganic fiber, etc., cashew dust, graphite and the like to cause less squeal noise or less groan noise.
However, required performances to the brake in recent years are tend to be increased more and more, in particular, further improvement in effectiveness of the braking and noise and vibrations (groan noise, squeal noise, vibrations, etc.) are required. In the disc brake pads comprising the above-mentioned NAO material, it is required to improve effectiveness of braking under high load, high speed, high temperature, etc.
Also, when a driver is to decrease a speed of a car by pedaling a brake pedal, if there is a tendency of increasing effectiveness of braking with a lapse of a braking time (hereinafter referred to as “a friction coefficient (μ) build up”) even when a pedaling power is made constant, it is preferred since the driver feels secure.
However, when a case in which, for example, braking is carried out with a constant pedaling powder from an automobile speed of 50 km/h is considered, it is preferred to exist μ build up during braking from a car speed of 50 km/h to about 20 km/h. On the other hand, during a car speed of 20 km/h to immediately before stop of 0 km/h, the μ build up is considered to be preferably moderate or none in view of expecting inhibition of occurrence of groan noise or squeal noise.
Here, there is a means to add a polishing agent having higher Mohs' hardness to the conventional disc brake pad comprising a NAO material to obtain μ build up. In such a case, the μ build up continues immediately before stop, so that there is a tendency of easily occurring groan noise or squeal noise.
Moreover, one of the problems in recent years with regard to the above-mentioned noise and vibrations, when an automobile is allowed to stand for more than several hours and braking is carried out after the brake was cooled to an atmospheric temperature and moisture-adsorbed, effectiveness of the brake is easily increased by braking with the initial several times (hereinafter referred to as “μ increase after allowing to stand”). At that time, there are cases in which squeal noise occurred (hereinafter referred to as “cold squeal noise”), and this is required to be improved.
In FIG. 1 and FIG. 2, μ build up is shown in the relationship between a braking time and μ. Of these, FIG. 1 shows μ build up of a conventional product, and FIG. 2 shows μ build up in an ideal form. In both of FIGS. 1 and 2, increase in μ relative to a braking time is observed, but in FIG. 1, μ build up continues immediately before stop, and there are tendency of easily causing groan noise or squeal noise. To the contrary, in FIG. 2, during an automobile speed of 20 km/h to immediately before stop of 0 km/h, the μ build up shows moderate behavior and occurrence of groan noise or squeal noise is a little.